The growing increase in the speed of ships and in the frequenting of certain maritime routes renders evermore obvious the problem posed by the drifting on the sea of floating objects which may be located on the route of ships. To be of some effectiveness, the detection of these objects and their location must be early enough to possibly allow the crew of-the ship to alter the heading so as to avoid these objects. Insofar as the ship is moving, the important parameter is here the relative position of the object detected with respect to the ship. It may also be useful to know in a relatively precise manner if the detected object is floating on the surface or else if it is submerged, its submersion depth.
To perform the detection of floating objects there exist several types of devices using mainly electromagnetic sensors such as radars, optical sensors using visible or infrared wavelengths or else laser sensors. These devices are effective for detecting floating or very slightly submerged objects, in particular when the sea is fairly calm. On the other hand, if the floating object is of a relatively small size, and if the sea is heavy the effectiveness of the electromagnetic sensors very substantially decreases. Likewise their effectiveness decreases rapidly if the depth of submersion of the object increases, as in the illustration of FIG. 1. The effectiveness of the electromagnetic sensors 10 is moreover affected by the incidence of the direction of emission with respect to the surface of the water. Specifically, when we seek to increase the detection range so as to adapt it to the requirements of high-speed ships, the electromagnetic wave 11 is often transmitted in a grazing manner with a small angle of incidence with respect to the surface of the sea, the angle of incidence then becoming close to the limit angle corresponding to the total reflection on the surface of the sea of the transmitted wave. In a rough sea, the performance of these devices is furthermore impaired by the waves.
To alleviate the problems related to electromagnetic sensors, it is known to use acoustic sensors 12, such as active sonars, which make it possible to detect obstacles, including submerged objects 13. However, the utilization of the properties of the acoustic waves such as it is carried out in the devices known to the prior art, is unsuitable for high-speed ships. Specifically, these ships exhibit particular structural characteristics such as in particular the existence of a hull made of several very slender floats. Moreover, the means for detecting these high-speed ships undergo the nonnegligible influence of the Doppler effect engendered by their own speed.
For high-speed ships, the detection and the location of objects must then involve several parameters such as the emission frequency of the acoustic waves, the shape of the transmitted waves, the dimensions of the antenna, and the Doppler effect consequent upon the speed of the ship. These various parameters are not systematically used by the existing devices.
It is known that for a linear antenna for example, the angular resolution of the measurement performed is given by a formula of the type:θ3=k.λ/L   [1]where:                θ3 is the aperture of the main lobe of the antenna at −3 dB,        λ is the wavelength of the acoustic wave given by λ=c/f, where c represents the propagation speed of the wave in the medium considered (sea water or fresh water for example) and f the frequency of the acoustic wave,        L is the length of the antenna,        K is a coefficient in particular related to the form of the antenna and to the weighting function used for the lobes. k can in particular take a value lying between 0.9 and 1.5.        
Relation [1] demonstrates that θ3 is dependent on the frequency of the transmitted wave, as well as the length of the antenna.
For a high-speed ship, it is desirable to have at one and the same time a fairly long range, and a sufficient angular accuracy so as to be able to determine in an early and accurate manner the position of the objects present on the route of the ship.
Satisfying the accuracy requirement prompts one to choose, in accordance with relation [1], a relatively high emission frequency associated with an antenna of large size. However, it is known that the absorption coefficient of acoustic waves is dependent on the frequency of the transmitted wave or more precisely on the inverse of the square of the frequency. Stated otherwise, the higher the transmission frequency is and the more the rangeis limited, the transmitted power being otherwise constant. The range requirement therefore leads to a choice of emission at relatively low frequency. The duality of these two requirements culminates finally in the search for a compromise.
In the particular case of a multihull ship, the compromise turns out to be more difficult to find than for a conventional ship. The hydrodynamic constraints of such craft make it essential in particular to reduce anything that may affect the drag of the hulls and in particular the size of the antennas. The slenderness of the floats does not furthermore make it possible to have available an antenna of satisfactory size to ensure the desired directivity. It is possible nevertheless to alleviate this handicap by using a system of antennas of low dimensions comprising for example, at emission or at reception, two or more antennas. Each antenna can be placed on a distinct hull. An array of antennas is thus produced. It is necessary however in this case to take into account the occurrence, in addition to the main lobe, of spurious image lobes. These spurious lobes also called array lobes are induced by the distance which separates the floats on which the antennas are disposed. This spacing, very large compared with the wavelength of the acoustic signal, leads to a spatial undersampling of the received signal which induces the occurrence of the array lobes. The result is the existence of an ambiguity as to the direction of arrival of the signal back-scattered by a floating object.
The detection of floating objects by high-speed ships is furthermore affected by the Doppler effect which intervenes in a nonnegligible manner in the propagation of the signals and the reception of the echos. The Doppler effect must be taken into account if one wishes to undertake correct determination of the position of floating objects.
For a ship carrying A transmitter and a receiver, the transmitter transmitting a wave reflecting on a floating object, subjected to a simple drift motion due to the currents or to a relatively weak wind, the Doppler effect is simply related especially due to the displacement of the ship. In this case, it is possible as a first approximation to write:freceived≅ftransmitted(1+(2/c).vship·cos g)   [2]
where g represents the bearing in-which the object lies with respect to the heading of the ship. The bearing is determined with respect to the direction of the speed vector of the ship which is taken as reference. It is also the angle relative to the axis of the antenna.
In expression [2], the Doppler is represented by the expression (2/c).vship·cos g. This quantity characterizes the frequency slip observed on reception with respect to the transmitted frequency, slip due to the Doppler effect. The coefficient 2 results from the fact that the wave transmitted by the sonar situated on the ship and reflected by the submerged object undergoes the Doppler effect on the outward-return path.
For a ship comprising an transmitter and a receiver placed at the same location on the ship, the effect of the motion of the ship manifests itself by a difference of duration between the path traveled by the acoustic wave transmitted between the transmitter and the floating object, and the path traveled by the acoustic wave reflected between the floating object and the receiver. This difference of duration is mainly due to the displacement of the ship during the propagation of the wave. If the ship is approaching the return path turns out shorter. Conversely, if the ship is receding this path turns out to be longer.
The Doppler effect manifests itself on the signal, according to each case, by a compression or a dilatation of the time, in the identical ratio v/c on the outward leg and on the return leg of the wave. With active sonar the estimation of the Doppler is generally carried out by performing the inter-correlation of the received signal with models of the transmitted signal which are affected by a Doppler effect. This estimation can also be carried out by an interspectral analysis, by considering that for certain signals, as was signaled above, the measurement of a frequency slip may be sufficient itself. It is also possible to measure the compression of the time between two pulses received with respect to the interval between the two corresponding transmissions.
The detection of objects and the accurate determination of their positions are often complicated by the reverberation originating from scatterers distributed in the space and which will affect the useful signal to reverberation ratio. This reverberation will be all the more annoying the swifter the ship moves and the larger the spatial undersampling thereby causing spurious lobes.